Surveillance is often described as providing information for action. As human beings we can only function effectively by constantly monitoring what is happening around us, using our senses to provide information that will enable us to respond (take action) to external threats and opportunities, or to plan future actions. Although the amount of information we have to process varies according to different situations, in most situations we need to be able to monitor what is happening in our environment on a continuous basis, so that we can act in as immediate a way as possible. There are analogies to this need for continuous monitoring of events or circumstances in order to act effectively in most areas of human activity, such as the need for banks to monitor financial transactions and trends in the economy, and the need for public health bodies to monitor trends in hazards, exposures and health events in order to protect the health of populations and individuals.
While the above analogy may be relatively simplistic, it emphasises the fact that a reliable supply of timely, accurate and relevant information is essential to almost everything we do. The control and prevention of infectious diseases and other environmental threats to health is no different in this respect. The processes by which much of the information that is used to inform public health action is collected, analysed and disseminated to those who need to know are collectively known as surveillance. Without good surveillance, public health action is unlikely to be effective or efficient.
The general definition of surveillance, as given in the Oxford English Dictionary is:
n. Close observation, especially of a suspected spy or criminal
ORIGIN C19: from Fr., from sur- 'over' + veiller 'watch'
While this definition makes explicit the observational aspect of surveillance, for many members of the general public the link with crime and espionage has negative associations, while for public health practitioners it does not provide any sense of purpose. Fortunately, however, there are many definitions of surveillance to be found in epidemiological texts, one of the earliest was drawn up by Alex Langmuir, the first chief epidemiologist of the Communicable Disease Center, now the Centers for Disease Control and Prevention in the United States:
"Continued watchfulness over the distribution and trends of incidence through the systematic collection, consolidation and evaluation of morbidity and mortality reports and other relevant data together with the timely and regular dissemination to those who need to know"
This definition emphasises many of the key attributes of surveillance: the need for surveillance processes to be continuous and systematic; the fact that surveillance may, and often needs to, draw on data from many different sources; and the need for regular and timely outputs.
Last, in his Dictionary of Epidemiology, also provides an indication of purpose in his definition of the surveillance of disease:
"The continuing scrutiny of all aspects of occurrence and spread of disease that are pertinent to effective control" 
Statistics on morbidity, and more particularly mortality, have been produced in many societies over many hundreds of years, such as in the case of the 'Bills of Mortality' that were produced and published in London. The London Bills of Mortality were devised in the early sixteenth century in London. The information was collected by the Parish Clerk's Company of London, and published weekly. Monthly and Yearly digests were also issued. Over time the detail provided by the bills increased. Initially they contained only burials, but by the 1570s the total number of baptisms was also returned. In 1629 cause of death information was given and in the early eighteenth century the returns began supplying a distribution of the ages at which Londoners died. Although John Graunt published a statistical analysis of the Bills as Natural and political observations made upon the bills of mortality in 1662, their production and publication was not directly linked to public health action, and as such they are not generally cited as examples of surveillance (although it is known that Londoners bought copies of the bills and scanned them for signs of impending disease, particularly plague, and may have made their own plans to leave the city at the first sign of rising numbers).
The first exponent of the systematic analysis of official medical statistics for the purposes of monitoring disease and identifying associations between disease and demographic groups is generally held to be William Farr. Farr was the first compiler of scientific abstracts in the General Register Office in London, where he set up a system for routinely recording the causes of death in England and Wales. He collected, analysed and interpreted vital statistics, and plotted the rise and fall of epidemics of infectious disease. He published his results in weekly, quarterly and annual reports. For example, for the first time it allowed the mortality rates of different occupations to be compared. In addition, in 1864 Farr was the first to publish work containing material calculated and printed by a machine, Scheutze's Difference Engine, which was a forerunner of the computer.
1. Last JM. A Dictionary of Epidemiology (Second Edition). Oxford University Press 1988, New York.